The present invention relates to metastable aluminium-titanium materials and to a process for their manufacture.
In recent times the continuing and rapid advancement in technology has led to the design and development of high performance devices capable of operating in increasingly hostile service conditions. From the existing trend in the technological development it can be anticipated that the severity of the service conditions that these devices will have to withstand will increase in the time to come. One of the important criterion to realize the continuing improvement in the efficiency of these devices is the development of materials that will be used to make these devices and their ability to withstand increasingly hostile service conditions. Aluminium alloys with titanium as the main alloying element are one such class of materials actively pursued for possible application in relatively high temperature and high stress conditions.
The use of titanium in aluminium has been explored as an alloying element so as to synthesize high performance materials for advanced engineering applications. Processing techniques based on molten metals (such as conventional casting and spray atomization and deposition) and metallic powders (such as powder metallurgy and mechanical alloying) have been investigated. Amongst these techniques, the synthesis of aluminium-titanium materials can be carried out more cost effectively by the methods based on molten metals. The studies conducted so far have shown that the addition of titanium in the liquid aluminium leads to: a) an increase in the melting temperature of aluminium significantly and b) reaction between titanium and aluminium to form Al3Ti through peritectic reaction. For example, addition of 10 weight percent of titanium in aluminium raises the melting point of aluminium from xcx9c660xc2x0 C. to xcx9c1200xc2x0 C. besides significantly increasing the chemical reactivity of the resultant molten Alxe2x80x94Ti mixture. This necessitates the use of more expensive and specialised furnaces (such as induction furnace) capable of heating to higher temperatures and crucibles made of highly inert materials thus increasing the overall cost of synthesis material. The interaction between titanium and aluminium in the molten condition results in either a solid solution of titanium in aluminium (with very limited solid solubility) or the formation of Al3Ti through peritectic reaction or primary solidification. The microstructures of aluminium-titanium synthesized using conventional casting with slow cooling rate and spray atomization and deposition with reasonably high cooling rates (xcx9c103-4 K/s) revealed the as-expected existence of Al3Ti intermetallic phase and equilibrium/extended solid solubility of titanium in aluminium. Neither of these techniques has shown the capability of synthesizing aluminium-titanium materials at temperatures lower than that exhibited by equilibrium Alxe2x80x94Ti phase diagram and in retaining the titanium as titanium in its elemental form following solidification by controlling the reaction between the molten aluminium and titanium.
We have now found a process for producing metastable aluminium-titanium materials which may be successfully produced at temperatures lower than those required by the equilibrium Alxe2x80x94Ti phase diagram. The metastable aluminium-titanium materials have been produced by controlling the reaction of the titanium with the molten aluminium in order to retain a proportion of the titanium in its elemental form.
According to the present invention there is provided a process for the manufacture of a metastable aluminium-titanium material comprising the steps of:
i) melting aluminium in a crucible;
ii) mixing solid particulate titanium with the molten aluminium; and
iii) disintegrating or spraying the molten mixture on a metallic substrate such that the molten mixture is deposited and solidified on the metallic substrate,
and wherein at least a substantial portion of the titanium is retained as elemental titanium after the molten mixture is solidified on the metallic substrate.
There is also provided a metastable aluminium-titanium material manufactured by the process described in the immediately preceding paragraph.
It will be understood that by the term xe2x80x9cat least a substantial portion of the titanium is retained as elemental titaniumxe2x80x9d it is meant that sufficient elemental titanium is present in the metastable titanium material to strengthen the aluminium matrix. The presence of titanium in the matrix advantageously serves to improve the mechanical behaviour of the aluminium at ambient and elevated temperatures as a result of its high melting point, strength and modulus properties. If not for the presence of metastable titanium in the matrix and associated metastable strengthening, the synthesis requirement for conventional Alxe2x80x94Ti alloys with similar amounts of titanium in the form of precipitates or in solid solution will necessitate a much more complex combination of melting process (skull melting), furnaces (induction furnace), crucible (titanium) and superheating temperatures. As an example, synthesis of Al with 10 wt. % Ti will require a superheat temperature of xcx9c1200xc2x0 C. when compared to 750xc2x0 C. using the presently described process. It is preferred that the elemental titanium retained in the metastable aluminium-titanium material is present in an amount ranging up to 20% by weight of the material.
It is preferred that the retained elemental titanium in the particulate form is uniformly distributed throughout the material. The uniform distribution indicates a non-aligned and non-clustered (as much as possible) distribution in three directions (an isotropic distribution).
In a preferred embodiment, the mixture of molten aluminium and particulate titanium is poured or allowed to flow from the crucible, and is subsequently disintegrated using jets of inert gas, the spray from the disintegrated mixture being deposited and solidified on the metallic substrate.
The at least a substantial portion of the titanium may be retained as elemental titanium by controlling the exposure time of the titanium to the molten aluminium. The period will be readily determined by simple experimentation having regard to the temperature of the aluminium to which the titanium is exposed and the particle size of the solid particulate titanium. The lower the temperature of the molten aluminium, the longer the treatment period which the titanium may be exposed to the hot aluminium. The larger the particulate size of the solid particulate titanium, the longer the treatment period which the titanium may be exposed to hot aluminium. The emphasis however is to advantageously minimize the exposure time of the particulate titanium to the molten aluminium so as to avoid undesirable reactions between the two and to enhance the retention of the elemental titanium by aluminium following solidification.
The solid particulate titanium may alternatively or additionally be treated prior to mixing with the molten aluminium in order to increase the period the titanium may be exposed to the hot aluminium and to enhance the retention of elemental titanium in aluminium following solidification. For example, the solid particulate titanium may be treated to produce an oxide coating thereupon so as to decrease the reactivity of the solid particulate titanium to the molten aluminium. Titanium powders may be preheated to temperatures in the range of from 600xc2x0 C. to 815xc2x0 C., preferably about 650xc2x0 C. for a period of at least 30 minutes, preferably 1 hour in order to produce an oxide surface layer.
The present invention further provides a metastable aluminium-titanium material, of which a substantial portion of the titanium comprises solid particulate titanium which is substantially uniformly distributed throughout the aluminium.
The metastable aluminium-titanium material comprises the presence of nearly uniformly distributed elemental titanium particulates in a non-aligned and non-clustered form in all three directions, with minimal reaction with the aluminium based matrix. The materials do exhibit the presence of finite amount of Alxe2x80x94Ti based phases, and minimal amount of non-interconnected porosity. The presence of Alxe2x80x94Ti based phases is advantageously mostly confined to the near vicinity of the titanium particles.
Aluminium suitable for use as the matrix of the metastable aluminium-titanium materials include the aluminium based materials containing alloying additions such as copper, silicon, zinc, iron, magnesium either independently or in combination with each other.
Preferably the aluminium is treated prior to melting in order to eliminate surface impurities. A suitable method for treating the aluminium includes washing the aluminium with water and acetone.
In the process of the present invention the aluminium is melted in an inert crucible or other suitable container. The metal may, for example, be melted by resistance melting based techniques.
The molten aluminium is then held at a temperature for the blending of the solid particulate titanium. The superheat temperature is so selected so as to ensure the complete melting and sufficient fluidity of the molten metal so that it can be stirred easily and effectively.
The solid particulate titanium for use in the present invention preferably include ones with purity levels xe2x89xa799%. For the purpose of other than a binary Alxe2x80x94Ti system, the present methodology may incorporate any other titanium based particles in the size ranges containing average particle sizes of preferably  less than 200 xcexcm.
The solid particulate titanium may be combined with the molten metal by any convenient means. Typically the solid particulate ceramic material may be combined with the metallic metal by their additions while stirring the molten aluminium. The stirring is preferably done using a suitably designed stirrer being stirred in the speed range of 450 rpm-900 rpm and placed in the crucible below the surface of the melt. The stirrer design employed in the present study comprised a shaft having a length of about 24 cm and two blades pitched at about 45xc2x0 to the vertical, and having a diameter of about 0.6 D, where D is the diameter of the melt at rest.
The mixed blend, following the addition of the titanium particles, is immediately poured or allowed to flow from the crucible.
The mixed blend poured from the crucible is then preferably disintegrated. The poured molten mixture is most preferably disintegrated using jets of inert gas. Suitable inert gases for use in disintegrating the poured molten mixture include argon, and nitrogen. The jets of inert gas are advantageously aligned at 90xc2x0 to the axis of molten metal stream for best results. As a result of disintegration the stream of mixed blend is converted into a form of spray with the average droplet/splat size of about 180 xcexcm. The resultant disintegrated mixture thus obtained is subsequently deposited into a metallic substrate. Typical metals used for substrate include iron and copper based materials. The process advantageously allows the substrate to be used at ambient temperature thus enabling to minimize the cost of the process. The Alxe2x80x94Ti materials can be made in the dimensions suitable for structural applications at ambient and elevated temperatures and as control materials for synthesis of more dilute equilibrium Alxe2x80x94Ti materials using conventional techniques such as casting.
Preforms produced by the process of the present invention are advantageously near fully dense and in a near final shape with the matrix having a fine grained equiaxed microstructure. The preforms may be produced in near final product form requiring minimal amounts of machining.
The metastable aluminium-titanium material produced according to the present invention at temperatures of about 750xc2x0 C. are significantly lower than that predicted by the equilibrium phase diagram. The melting temperature of aluminium containing 6 weight percentage of titanium under equilibrium conditions will approach closely to 1100xc2x0 C. and will hence require even higher temperatures (at least a 50xc2x0 C. superheating) for processing through conventional molten metal methods. The material of the present invention retains titanium as elemental titanium in the microstructure following the solidification of aluminium. Furthermore, the low level of porosity which may be achieved indicates the feasibility of these methods to be used for near net shape synthesis. Finally, an increase in microhardness exhibited by aluminium-titanium materials synthesized using the present invention when compared to pure aluminium indicates that the presence of titanium in the aluminium matrix will favourably increase the mechanical properties of the resultant bulk aluminium-titanium material.